406 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



the other hand, if DNA having known biological activity (as deter- 

 mined by ability to transform the genetic constitution of a bacterium) 

 is used as a primer, both the product and the primer added end up 

 being inactive. Why this is so is not known, but it is strongly sus- 

 pected that the polymerizing enzyme added contains a small amount 

 of depolymerizing enzyme that breaks up DNA chains and thus de- 

 stroys activity. 



Again, the Kornberg synthesis does not prove that the hypothesis 

 is correct. It is just possible that an unkind nature could have 

 evolved a system that would do just exactly what the hypothesis 

 predicts but by a different mechanism. 



About the next question: How is genetic information translated? 

 How do we develop from that minute egg cell? These are enor- 

 mously difficult questions, and we know relatively little in detail about 

 the answers. They involve the whole of development, differentiation, 

 and function. There are working sypotheses — widely used and use- 

 ful ones — that suggest how some of the steps occur. 



We know that in our bodies there are many thousands of kinds of 

 protein molecules — large, long molecules made of amino acids and 

 very specific in their properties. One, for example, is hemoglobin. 

 It is built of 600 amino acids strung together in a particular way. 

 There are two kinds of chains of amino acids per hemoglobin mole- 

 cule, each in pairs, with each chain about 150 amino acids long. And 

 we know that there are segments of DNA — two, we postulate — in our 

 chromosomes that say how to build the two protein subunits. 



A widely used working hypothesis assumes that against a single 

 chain of DNA there is formed a chain of another kind of nucleic acid, 

 called ribonucleic acid or RNA. RNA like DNA, is built of four 

 nucleotides. The DNA code is translated into a complementary se- 

 quence of RNA. RNA then moves from the nucleus into the 

 cytoplasm. There it is incorporated into microsomes, submicroscopic 

 structures in which protein synthesis occurs. In the microsome, 

 RNA units are believed to serve as templates against wdiich amino 

 acids are lined up in proper sequence. 



Amino acids, derived from the proteins in our food, are first acti- 

 vated by enzymes and subsequently hooked to small caiTier segments 

 of RNA that serve to carry the amino acids to their proper places on 

 the microsomal RNA templates. 



For each amino acid there is a specific carrier RNA, made up of 

 about 80 nucleotides. Each of the 20 carrier RNA's contains a cod- 

 ing unit of 3 nucleotides presumably complementary to a 3-nucleotide 

 coding unit in the template RNA in the microsome. "Wlien all coding 

 units in the template are matched by their complements, the amino 

 acids are lined up in proper order along the template and are joined 



